GC-MS Simultaneous Determination of Enalapril and Enalaprilat in Human Plasma: Application to a Clinical Pharmacokinetic Study

Enalapril is a prodrug that is metabolized to its main active metabolite, enalaprilat that is a potent inhibitor of angiotensin converting enzyme (ACE). A simple, rapid, sensitive, precise and accurate SIM GC-MS method, using solid-phase extraction and derivatization with methyl iodide, for the quantitative determination of enalapril and enalaprilat in human plasma was developed and validated. The ions at m/z 220 for enalapril, m/z 234 for enalaprilat and m/z 172 for internal standard, perindopril, were monitored. The calibration graphs for the analytes were linear in the 5-160 ng mL(-1) concentration range (r > 0.999). Intra-day and inter-day precision for enalapril ranged from 2.4 to 3.5 and 3.9 to 7.9%, and for enalaprilat from 5.3 to 7.8 and 6.7 to 9.1%. Accuracy was better than 105.5% for both enalapril and enalaprilat. The method was sensitive with a lower limit of quantitation (LLOQ) of 2 ng mL(-1) for both analytes and was not interfered by other plasma components. The method was applied for the determination of enalaprilat in a pharmacokinetic study after single oral administration of 20 mg enalapril to 24 healthy subjects.